Thunnus orientalis, known commonly as the Pacific Bluefin Tuna,is a highly commercial and valuable species. Based on estimates of spawning stock biomass (SSB) it is estimated that this species’ population has declined between 19–33% over the past 22 years (or three generation lengths, using an estimated 7.4 year generation length).Based on several fisheries reference points, the most recent stock assessment concludes that overfishing is occurring and the stock is overfished (ISC 2014). The latest fisheries stock assessment estimates that the SSB in 2012 was approximately 4% of the stock’s estimated unfished SSB levels. Moreover, the average recruitment in the last five years may also be below the historical average level of recruitment, with the recruitment level in 2012 being the 8th lowest in 61 years. Additionally, there is concern that optimistically high levels of adult natural mortality and assumptions of no relationship between recruitment and biomass used in the most recent stock assessment may have contributed to an overly optimistic stock outlook. Furthermore, the current fishery may be dependent on one strong cohort, which will likely disappear given the targeting of juveniles in the Western Pacific (Maunder et al. 2014). For these reasons, this species is listed as Vulnerable under Criterion A2bd. Based on the current WCPFC and IATTC management and conservation measures, it is unlikely that the status of the stock will improve if low recruitment continues, therefore, more effective management measures to reduce fishing mortality and increase SSB are needed.

Thunnus orientalis comprises a single Pacific-wide stock that is found primarily in the North Pacific Ocean. Spawning has only been reported in the western North Pacific Ocean (Bayliff 1994, Chen et al. 2006, Tanaka et al. 2007). Although it is primarily a temperate water species, it also ranges into tropical waters. There are records of this species from New Zealand and French Polynesia. However, more information is needed about their distribution in these areas, and there is no evidence of spawning there.

The International Scientific Committee for Tuna and Tuna-like Species in the North Pacific Ocean (ISC) is responsible for conducting regular stock assessments for the Pacific Bluefin Tuna. The ISC created the Pacific Bluefin Tuna Working Group (PBFWG) in 1996 with the task to assemble, conduct and analyse fisheries statistics, biological studies and stock assessments for Pacific Bluefin Tuna. The West and Central Pacific Fisheries Commission (WCPFC) and the Inter-Tropical Tuna Commission (IATTC) are jointly in charge of the management of Pacific Bluefin Tuna, and use the ISC stock assessment determination and advice to establish conservation and management measures.

The most recent stock assessment states that overfishing is occurring and the stock is overfished based on several biological reference points commonly used by fisheries managers (ISC 2014). No target and limit reference points have been adopted officially by the WCPFC and IATTC, the two tuna regional fisheries management organizations (RFMOs) in charge of managing the Pacific Bluefin Tuna. Based on the 2014 ISC stock assessment base case model, the spawning stock biomass (SSB) of Pacific Bluefin Tuna has fluctuated substantially over time (1952–2012) with an overall negative trend. It is estimated that the SSB in 2012 was approximately 4% of the stock’s estimated unfished SSB levels. The recruitment level in 2012 was also estimated to be relatively low (the 8th lowest in 61 years) and the average recruitment in the last five years may also be below the historical average level of recruitment. Furthermore, the stock is experiencing high exploitation levels exceeding the majority of target and limit biological reference points used by fisheries managers (ISC 2014). Furthermore, a recent tagging study estimates a natural mortality (M) median value for Pacific Bluefin Tuna aged 5 years and above to be 0.15 year-1 (Whitlock et al. 2012). This natural mortality is lower than the value currently used in the ISC stock assessment (M=0.25 year-1). As lower natural mortality rates are associated with a less productive stock, the Pacific Bluefin Tuna may be more depleted than is presented in the ISC 2014 stock assessment.

In addition, there have been some concerns about the ISC stock assessment for Pacific Bluefin Tuna because the models do not adequately fit the data (Maunder et al. 2014). Yet, sensitivity analysis in the ISC models are robust to the general conclusions that the SSB is very depleted and overfishing is occurring (ISC 2014). An external analysis supported the management advice provided by the ISC stock assessment model for Pacific Bluefin Tuna, confirming that the species urgently needs new management action to ensure its sustainability (Maunder et al. 2014). The current high fishing mortality rates, including taking of juveniles, may not allow any new cohort to enter the spawning biomass. Substantial immediate cuts in fishing mortality of juveniles are needed to ensure the viability of the Pacific Bluefin Tuna, in addition to protection of spawning adults, at least until reductions in fishing mortality of juveniles allow for incorporation of new spawners (Maunder et al. 2014, ISC 2014).

Population declines calculated over a 22 year period (three generation lengths) based on SSB from 1991–2012

Since 1952, spawning stock biomass (SSB) of Pacific Bluefin Tuna has fluctuated substantially, and therefore two methods were used to estimate the change in SSB over the last 22 years (three generation lengths). In the first method, a 33.2% decline was calculated based on the linear percent change in SSB between 1991 and 2012 using a 3 year average for SSB endpoints (2012 average SSB 2010-2011-2012 divided by the 1991 average SSB 1990-1991-1992). However, since the SSB time series shows many non-linear fluctuations since the 1950s, this method is highly sensitive to the generation length estimate used for its calculation. For comparison, the average annual rate of change in SSB across the entire time series from 1952 to 2012 was calculated to be -0.93% per year; assuming an exponential pattern of decline (with average annual decline of 0.93%) over the past 22 years, this yields an 18.6% decline. Alternately, the average annual rate of change in SSB over the past 22 years (1991–2012) is -1.16% per year; this yields a 22.6% decline over the past 22 years. Therefore it is estimated that there has been at least a 19–33% decline over the past 22 years (three generation lengths for this species).

Summary of Population data and stock indicators based on the 2014 ISC Stock Assessment EvaluationCatch trends: Fisheries in the Pacific Ocean have targeted Pacific Bluefin Tuna for several centuries in the western Pacific and at least since the beginning of the 20th century in the eastern Pacific. Data prior to 1952 is of relatively poor quality and therefore fisheries data only from 1952 to 2012 is included in the latest stock assessment (ISC 2014). Since the 1950s, catches of Pacific Bluefin Tuna have fluctuated substantially over time and by gear time. The maximum historical catch occurred in 1956 with 39,824 t and the lowest historical catch occurred in 1990 with 8,588 t. During the last 10 years, the average catch has been 21,250 t, with most of the catch (80%) occurring in the western Pacific. Since the 1950s, the catches have been predominantly composed of juveniles, and since the 1990s, the catch of age 0 has increased significantly.

Spawning stock biomass (SSB) trends: Based on the 2014 stock assessment base case model, since 1952 the SSB of Pacific Bluefin Tuna has fluctuated substantially over time. The first year of the assessment, in 1952, the SSB was ~90,000 t. Estimates of SSB peaked at ~140,000 t in 1961, declined to ~24,000 in 1974, climbed back up to ~50,000 t in 1978, declined again to ~18,000t in 1984, climbed back up to ~87,000 in the 1995 and have fallen to about ~26,000 in 2012. It is estimated that the SSB in 2012 is approximately 4% of the stock’s estimated unfished SSB levels.

Recruitment: Recruitment estimates (age 0 fish) have fluctuated widely with no apparent trends between 1952 and 2014. An average recruitment of 15 million fish has been estimated for the entire period of the assessment. It has been observed that average recruitment in the last five years may have been below historical average levels. The recruitment level in 2014 was estimated to be the 8th lowest in 61 years.

Fishing mortality trends: From 1952 to 2014, the average fishing mortality for juvenile ages (0–3 ages) of Pacific Bluefin Tuna was higher than for adult ages (age 4+ fish). The more recent fishing mortality rates for ages 0-6 for the years (2009–2001) show an increase of 19% (Age 0), 4%, (Age 1), 12% (Age 2), 31% (Age 3), 60% (Age 4), 51% (Age 5) and 21% (Age 6) in respect to the fishing mortality rates for ages 0–4 in the years 2002–2004. The year 2002–2004 is the reference year for the most current WCPFC conservation and management measures in place.

Stock status determination: No target and limit reference points have been adopted officially by the WCPFC and IATTC, the two tuna RFMOs in charge of managing the Pacific Bluefin Tuna. Yet the ISC estimated several target and limit reference points commonly used by fisheries managers to determine the status of the stock. The SSB in 2014 relative to the unfished SSB is less than 6%. Furthermore, the ratio of current fishing mortality rates (2009–2011) relative to all biological reference points indicates that current fishing mortality rate exceeds all target and limit biological reference points (Fmax, F0.1, Fmed, F10%, F20%, F30% and F40% ), except for Floss. Based on these reference points, the stock is considered overfished and overfishing is occurring.

Future projections: The large majority of projections under different harvest scenarios and low recruitment scenarios estimate a low probability of reaching SSB benchmarks of 10% of unfished SSB (SSFf=0) within 10 years. Given recent average recruitment levels (2009–2013) may be lower than that observed before 2009, the WGPBF highlights the importance of considering the projections with low recruitment scenarios.

Thunnus orientalis is an epipelagic and oceanic species, but seasonally comes close to the shore. It tolerates wide temperature ranges and forms schools by size, sometimes with other scombrids. It is found to 550 m depth. It is a voracious predator that feeds on a wide variety of small schooling fishes or squid, and also eats crabs and other less sessile organisms (Collette and Nauen 1983).

The sex ratio is about 1:1. Longevity is at least 15 years (Hsu 2000) but probably extends to 26 years (Shimose 2009). Size at first maturity is 100–150 cm FL (50–60 kg) at an age of approximately 3 to 5 years (Tanaka 2006). The all-tackle game fish record is of a 411.6 kg fish caught off Three Kings Islands, New Zealand in 2014 (IGFA 2014). Spawning occurs between Japan and the Philippines in April, May, and June, off southern Honshu in July, and in the Sea of Japan in August. Batch fecundity increases with length, from about five million eggs at 190 cm FL to about 25 million eggs at 240 cm FL (Schaefer 2001, Sawadaet al. 2005, Chenet al. 2006). Larvae have been shown to react to biological (e.g. food, predation) and physical conditions (e.g. sea temperature, sea flow) during early life history stages (Satoh 2010; Satoh et al. 2008, 2013).

An unknown proportion of the juveniles, one and two year old fish, migrate to the Eastern Pacific, stay a few years, and then migrate back to the Western Pacific. Fluctuations in the catch in the Eastern Pacific are related to the proportion of migrants that come from the Western Pacific (Bayliff 1994). Recent tagging experiments in the western and eastern Pacific, using electronic archival tags, have documented the tracks and timelines of transpacific and regional migrations (Kitagawa et al. 2009; Boustany et al. 2010).

Recently it has been theorized that the three species of bluefins move to small, strongly convergent segments of energetically forced eddy structures, which although unproductive, allow spawning schools of sufficient size to generate sufficient numbers of larvae (Bakun 2013). This strategy may circumvent resident predator pits while exerting sufficient predatory loss on resident predators to facilitate this circumvention, and is an important consideration in management of spawning areas.

Generation Length: Based on the age class data including maximum age, natural mortality and maturity estimates of Pacific Bluefin Tuna used in the 2014 ISC stock assessment (ISC 2014), the stable age distribution of the reproducing adult population (or 1 generation length) is calculated to be 7.4 years. According to the ISC 2014 assessment, a maximum age of 20 years was modelled and used as an accumulator for all older ages. It was estimated that only 0.15% of the unfished cohort remains by the age 20. Moreover, the assessment used an age at 50% maturity of 4 years and an age of 100% maturity of 5 years. Natural mortality was also age-specific, as 1.6 at age 0, 0.38 at age 1, and 0.25 from age 2 to 20 years.

This is a highly valuable species that is important in international commercial fisheries. Bluefin tuna have the highest value for any tuna species for use as sashimi, and one fish can be valued at over $US 100,000 (Volpe 2005).

In the Northern Pacific, the most important fisheries for this species are longline, purse seine, and pole and line. Other gears such as troll, set-net, and hand-line gears can take substantial catches as well. The majority of fish are caught in purse seine fisheries (61%), followed by longline (11%) and troll (11%) (ISC 2014). Many countries harvest Pacific Bluefin Tuna, but Japan catches the majority, followed by Mexico, the USA, Korea and Chinese Taipei. The majority of the catches occur in the North Pacific Ocean, while catches in tropical waters and the southern hemisphere are relative low and sporadic.

Since the 1990s, the West Pacific Ocean purse seine fleet has had the largest impact on the Pacific Bluefin tuna stock compared with the west pacific coastal fisheries, the Eastern Pacific fisheries, and West Pacific longline fisheries. The catch composition of the West Pacific Ocean purse seine fisheries is made up largely of juvenile fishes.

In the Eastern Pacific, a considerable portion of the purse seine catch is transported to holding pens in the Ensenada region of Northern Baja California, Mexico for fattening and later sale as sashimi grade fish (IATTC 2008). Tuna ranching was evaluated by a Bi-National Scientific Team funded by the Packard Foundation (Zertuche-Gonzalez et al. 2008).

Since the early 1990s, the increase of catches of juveniles of age 0 has substantially increased; which may be an indicator of the increasing fishery pressure on this species (ISC 2014). The increasing value of this species may be another indicator of the increasing fishery pressure on this species. The price of the first big bluefin brought into the Tokyo Tsukiji market, an annual event intended to generate publicity, has substantially increased each year: $396,000, 341 kg (2011), $735,000, 269 kg (2012), $1,800,000, 222 kg (2013).

Tolower the probability of SSB falling further below its historical lowest levels, the WGPBF advice is tosubstantially reduce fishing mortality and catches for the whole range of juvenile ages. Given the average low recruitment levels for the last 5 years, the WGPBF advises that the risk of SSB falling to its lowest observed levels might increase. It advocates more conservation and management measures should be implemented by the WCPFC and IATTC to minimize this risk. The group emphasizes that based on the current WCPFC and IATTC management and conservation measures, an increase in SSB is unlikely in the short term under the average low recruitment scenario.

In the Western and Central Pacific, the WCPFC has currently adopted Conservation and Management Measure (CMM) 2012-06 for Pacific Bluefin Tuna. In general, the CMM states that fishing effort for Pacific Bluefin shall stay below 2002-2004 levels except for artisanal fisheries, and that catches of juveniles (ages 0-3) shall be reduced below 2002-04 levels, except for Korea. In accordance with this CMM, Japan has instituted several catch limits on different components of the Japanese purse fishery. In 2013, the WCPFC Northern Committee has drafted a new CMM for adoption by WCPFC in December 2013. The draft CMM removes the exceptions for artisanal fisheries as well as Korean fisheries, and states that juvenile catches shall be reduced 15% below 2002–2004 levels. However, it remains unclear if the draft CMM will be adopted by WCPFC due to a reservation expressed by Korea on the removal of the exception for Korea.

In the Eastern Pacific Ocean (EPO), the IATTC has adopted Resolution C-13-02 for the conservation and management of Pacific Bluefin Tuna. In general, the resolution states that the total annual commercial catch in 2014 shall not exceed 5,000 metric tons and notwithstanding the 5,000 metric ton catch limit, countries with historical EPO catches of Bluefin Tuna may in addition catch up to 500 metric tons of commercial Pacific Bluefin Tuna catch. Mexico and the US are the two countries with historical records of substantial Bluefin Tuna catches in the EPO. Mexico has quotas in place to limit the amount of fish that can be caught by individual fish ranching operations. In recent years the US catch for this species has substantially declined and historically high catches for U.S. flag vessels were due to fishing activities in Mexican waters, as evidenced by recent catches by vessel flag (IATTC 2013).